This invention relates to the lamp arts. More particularly, this invention relates to a reflector coating and a method of preparation thereof for use in reflector lamps wherein a light source is contained in a housing having a transparent section and a reflective section, the reflective section being positioned to reflect a preponderance of generated light through the transparent section.
The reflector lamps of the present invention are particularly well suited for use in spot lighting, head lamps, and the like. Examples of typical reflector lamps include General Electric""s PAR 38 and PAR 64 lamps. PAR is the commonly accepted acronym for xe2x80x9cparabolic aluminized reflectorxe2x80x9d. Other commercially available reflector lamps may also benefit from aspects of the present invention. U.S. Pat. Nos. 3,010,045; 4,021,659; 4,804,878; 4,833,576; 4,855,634; and, 4,959,583 describe reflector lamps and methods of their manufacture, many of which may be modified in accordance with this invention.
A recent area of emphasis in reflector lamp design has been to increase energy efficiency. Energy efficiency is typically measured in the industry by reference to the lumens produced by the lamp per watt of electricity input to the lamp (LPW). Obviously, a lamp having high LPW is more efficient than a comparative lamp demonstrating a low LPW. In this regard, it is expected that governmental regulations will require a significant improvement in reflector lamp LPW in the near future.
One of the most commonly used reflector coatings is aluminum film, which is deposited on the surface of a reflector by thermal evaporation and sputtering. Manufacture costs are low and the film is stable at lamp operating temperatures over the life of the lamp. Reflectivities of the film in the visible spectrum are such that PAR 38 lamps incorporating the aluminum films are able to convert about 70% of the light emitted from the lamp filament tube to luminous output.
Silver films have a higher reflectivity and are used in optics, electronics, and in lighting. For the same PAR 38 example, silver-coated lamps are able to convert about 80-85% of the light emitted from the lamp filament tube to luminous output, a 15% lumen gain is thus expected.
Conventional manufacturing methods for assembling lamps with aluminum films incorporate several high temperature processes, including pre-heating, tubulating, aluminizing, brazing, and sealing. In the preheating step, the reflector is heated to about 800xc2x0 C. In the tubulating step, ferrules and an exhaust tube are welded to the base of the reflector. The reflector is then aluminized to provide the aluminum coating. Brazing involves the welding of light source to the ferrules. In the sealing step, a transparent cover lens is sealed over the reflector opening. Typically, an open natural gas and oxygen flame is used to carry out many of these heating steps. The flame heats adjacent portions of the reflector to high temperatures. In sealing, for example, the reflector and coating are subjected to a temperature of around 1000xc2x0 C. in the seal region, and around 650xc2x0 C. away from the seal.
Silver films may be prepared in a similar manner to the aluminum films. However, evaporated or sputtered silver films are notoriously unstable at temperatures in excess of 200 degrees Celsius. Silver films are readily oxidized at the temperatures used in sealing and the optical properties of the films destroyed. Unprotected silver films are thus unsuited to lamp manufacture by such processes. Moreover, the films exhibit poor chemical resistance to sulfide tarnishing, and thus the properties of the unprotected films are destroyed on exposure to the atmosphere.
Protective coatings of silicon dioxide on silver films are known for mirrors in optical applications. However, when sputtering is used to form a silicon dioxide film, oxygen introduced to the vacuum chamber for formation of the silicon dioxide film may take its ion form due to the high electric field within the chamber. The oxygen ions tend to attack the silver film prior to deposition of the silicon dioxide coating. As a result, the silver film becomes oxidized and its high reflectivity is lost. In extreme cases, the silver film becomes blackened and thus ineffective.
Another problem with forming silicon dioxide films on silver is that the silicon dioxide film, as deposited, is oxygen deficient (i.e., has a composition SiOx, where 1xe2x89xa6xc3x97xe2x89xa62). The refractive index of SiOx is larger than that of SiO2. Such oxygen deficient SiO2 on the silver film reduces the reflectivity of the protected silver film. As a result, the lumen output decreases.
Accordingly, there is a need in this art to develop a more energy efficient reflector lamp, which maintains acceptable light temperatures, light colors, life, and compatibility with current hardware.
In an exemplary embodiment of the present invention, a method of forming a lamp is provided. The method includes providing a reflective interior surface, consisting of the steps of providing a layer of silver, providing a protective layer which protects the silver layer against oxidation and sulfide formation, and providing a buffer layer intermediate between the layer of silver and the protective layer which protects the silver layer from oxidation during the step of providing the protective layer. The method further includes forming the lamp from the interior surface, a light source and a lens.
In another exemplary embodiment of the present invention, a lamp is provided. The lamp includes a light source within a housing having a reflective interior surface consisting of a protective layer disposed over a layer of silver. The lamp is produced by a method which includes annealing the reflective surface to increase its reflectivity.
In another exemplary embodiment of the present invention, a lamp is provided. The lamp includes a housing, a light source within the housing. A reflective interior surface includes a silver layer, a protective layer disposed over the silver layer. A lens closes the housing.
In another exemplary embodiment of the present invention, a lamp is provided. The lamp includes a light source within a generally parabolic housing having a reflective interior surface comprising a protective layer covering a silver layer. The light source has a longitudinal axis disposed substantially on the axis of said parabolic housing.
One advantage of this invention is the provision of a new and improved reflector lamp having superior LPW.
Another advantage of the present invention is the provision of a protective coating on a silver reflector.
Another advantage of the present invention is the provision a stoichiometric silicon dioxide coating with high reflectivity.
Additional advantages of the invention will be set forth in part in the description, which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.